This invention relates in general to a monolithically integrated circuit for an IGFET constant current source. The circuit of an IGFET constant current source containing a source-drain series arrangement of two IGFET's (insulated-gate field-effect transistors) of one conductivity type in series between the voltage supply and the substrate, in which circuit the gate electrode of the load IGFET of the series arrangement together with the first pole of the voltage supply and the common connection point of the two IGFET's is applied to the gate electrode of a further current source IGFET, by which the current to be switched constant, flows from or to the substrate is known from the German Published Patent Application (DE-OS) No. 25 02 689.
In this conventional IGFET constant current source the standard deviation values of the threshold voltages of the IGFET's are brought to a minimum in that there is acted upon the ratios of the width W to the length L of the channel regions. In this IGFET constant current source, however, the action of the substrate effect upon the threshold voltage has not been taken into consideration, and insulated islands would be necessary for avoiding the substrate effect.
This invention is based on the recognition that the threshold voltage ##EQU1## for P-channel IGFET's and ##EQU2## for N-channel IGFET's are chiefly subject to the variations of the surface charge density Q.sub.SS. In the equations (1) and (2), as well as in the following equations the parameters have the following meanings:
Q.sub.SS =Surface charge density ##EQU3## the mutual conductance constant, with
W=width of the channel region, and
L=length of the channel region,
C.sub.ox =specific capacitance of the gate electrode,
Q.sub.B =.sqroot.2.epsilon..sub.s qN.multidot.2.phi..sub.F =space charge,
U.sub.DD =supply voltage,
U.sub.Tn, U.sub.Tp =d.s. threshold voltages,
.DELTA.U.sub.T =variation of the threshold voltage owing to the substrate effect caused by variations in the surface charge density,
.phi..sub.M iSi=difference in the work functions between the gate electrode and the self-conducting silicon,
N.sub.n, N.sub.p =original substrate surface doping concentration ##EQU4## and
N.sub.i =intrinsic charge density.